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The Origin of the Earth
Hugh Rollinson
University of Gloucestershire
August 2001
Funded by the National Subject Centre for Geography, Earth & Environmental
Sciences ‘Earth Science Small Learning & Teaching Award’
Table of Contents
About the Author – Hugh Rollinson ............................................................................. 1
Introduction .................................................................................................................. 2
1 The Origin of the Universe and the Big Bang .......................................................... 2
1.1
An Expanding Universe ............................................................................. 2
1.2
The Cosmic Microwave Background ......................................................... 5
1.3
The Early History of the Universe .............................................................. 7
2 Star Formation......................................................................................................... 9
2.1
'Lumpiness' of the Universe ....................................................................... 9
2.2
Gravitational Collapse and the Formation of Stars .................................... 9
2.3
The Evolutionary Cycle of Stars............................................................... 10
2.4
Origin of the Chemical Elements ............................................................. 12
3 Planetary Formation and the Origin of the Earth ................................................... 15
3.1
The Planets of the Solar System ............................................................. 15
3.2
The Stages of Planetary Formation ......................................................... 15
3.3
The Search for Planets in other Solar Systems ....................................... 17
3.3.1
Finding Extrasolar Planets .......................................................... 17
3.3.2
Discoveries to date...................................................................... 18
3.3.3
For further information:................................................................ 18
4 And Finally............................................................................................................. 19
4.0.1
Reading and Further information ................................................ 19
4.0.2
Other useful websites.................................................................. 19
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About the Author – Hugh Rollinson
Hugh Rollinson holds a personal chair in Geology and is a member of the Geography
and Environmental Management Research Unit (GEMRU) at Cheltenham and
Gloucester College of Higher Education, where he has responsibility for projects
relating to Mineral Resources and Geochemistry. Hugh also runs GEMRU's
Geochemistry Laboratories which house the newly installed ICP spectrometer.
Hugh's long-term career interest is the early history of Planet Earth and much of his
work has centred on the petrology and geochemistry of Archaean rocks. After
graduating he was employed for nearly four years as a geologist with the geological
survey of Sierra Leone mapping Archaean greenstone belts and basement gneisses.
This was followed by PhD studies at Leicester University on the geochemistry of the
Lewisian Gneisses of Scotland. A two-year post-doc at Leeds took Hugh back to
West Africa and provided the chance to follow up geochemically work started in his
Survey days. A teaching position at Cheltenham followed, broken by a spell (19901993) when he was appointed associate Professor of Geology at the University of
Zimbabwe and subsequently chairman of the Department. More recently Hugh has
been working in west Greenland in the Isua Greenstone Belt on the metamorphic
petrology of 3.8 Ga sediments and volcanic rocks. Along with research interests in
the Archaean he also has research interests in mineralisation, mathematical geology
and geological pedagogy.
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Introduction
This module is about the origin of the Earth, and is in three parts. The first part is
about how we think the Universe formed, and the evidence for the Big Bang
hypothesis for the origin of the Universe. The second part of the module is about
galaxies and stars — how they form and how they are able to generate the chemical
elements. In the third part of the module we consider the origin of planets and how
we think our own planetary system formed.
Many of the processes we are considering took place a very long time ago. In the
text that follows time is referred to in millions of years (Ma - 106 yrs) or billions of
years (Ga - 109 yrs).
1
The Origin of the Universe and the Big Bang
The most popular theory for the origin of the Universe is the Big Bang Theory,
according to which the Universe expanded to its present enormous volume from an
initial miniscule starting volume. This expansion has taken place over the past 10 or
so billion years.
Two observations, both made during the 20th Century have profoundly shaped the
way in which we think about our Universe and have lead to the Big Bang Theory for
the origin of the Universe. First was the discovery by Edwin Hubble that our
Universe is expanding. Second was the discovery made in 1965 by Arno Penzias
and Robert Wilson that our part of the Universe is filled with microwave radiation.
This radiation has become known as the Cosmic Microwave Background (see
Section 1.2).
1.1
An Expanding Universe
It was during the 1920's that Edwin Hubble provided the first evidence that we live in
an expanding Universe. Hubble discovered that there is a simple relationship
between the distance to a remote galaxy and the redshift in the spectral lines from
that galaxy. This redshift is know as the cosmological redshift (see Box 1). Hubble's
observations showed that the greater the distance to a galaxy, the greater the
redshift in its spectral lines. These measurements strongly indicated that galaxies
appear to be moving away from us with speeds proportional to their distance. This
observation is made in whatever direction you may look in the sky, giving the (false)
impression that our solar system is at the centre of the Universe. The net effect of
this motion is that as time goes on the galaxies are getting further and further apart.
Astronomers describe this as an expanding universe.
Box 1:
The Cosmological Redshift
Astronomers routinely analyse the light from stars using the technique of
spectroscopy. Spectroscopy subdivides light into its constituent colours — violet to
red — and identifies from the presence of spectral lines, the chemical elements
which are present in the star. These are the elements which are involved in nuclear
reactions in the star. Each element is identified by a characteristic pattern of spectral
lines.
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As long ago as 1914 astronomers began to make a series of observations which led
to the conclusion that distant stars had spectral lines which were shifted from their
expected wavelengths in the spectrum, to positions which were at higher
wavelengths towards the red end of the spectrum. Using observations well known
from physics on the effects of relative motion on wavelength — the Doppler effect —
the red shift in starlight is interpreted as the motion of the galaxy away from the
observer.
The more distant a galaxy, the greater the redshift. Until recently the greatest
redshift was 5.64. However, new data reported in the journal Science in 1999 reports
galaxies with a redshift of 10. These formed when the Universe was only 9% of its
current size and probably just a few hundred million years old.
Edwin Hubble made the very important observation that there is a simple relationship
between the amount of red shift of a galaxy and its distance from earth. [See
http://www.astro.ucla.edu/~wright/cosmo_01.htm.] This discovery, published in 1929
and now known as Hubble's law, can be expressed by the relationship:
v = rH0
The velocity (of a galaxy away from Earth) = the distance x a constant (known as the
Hubble constant).
The precise value of the Hubble constant has been the subject of much debate, but
has been determined with increasing precision since the 1920s. Recently the value
was reported in the top American journal Science (1999, vol. 284, pages 1438-1439)
as 70 km/sec per megaparsec (a megaparsec is 3.26 million light years).
The importance of the Hubble constant is that it expresses the rate at which the
Universe is expanding. For this reason it is possible to calculate the age of the
Universe (see Box 2) from the Hubble constant. Clearly astronomers are keen to
determine the Hubble constant with the greatest of precision.
A very simple analogy may be drawn with the blowing up of a balloon on which a
number of reference points have been marked. The balloon is illustrative of the
Universe and the reference points galaxies. As the balloon is inflated each reference
point moves further away from the other, so from any reference point within the
balloon every other reference point appears to be moving away from the observer.
The observer does not have to be at the centre of the balloon to make this
observation. Where this analogy breaks down is that, on the skin of the balloon the
reference points will expand, as well as the distance between them. In the Universe
the expansion takes place in the space between the galaxies. The galaxies
themselves do not expand.
A very important consequence for observations which lead us to infer an expanding
Universe is that at some point in the past the matter which is becoming increasing
distant must have been concentrated all in one place. Astronomers believe that at
this point in time, the beginning of the Universe and the beginning of time, all the
matter of the Universe was concentrated in an infinitely small volume and was in a
state of infinite density. This time can be calculated by a number of different
methods. [For further details see Box 2.] The huge explosion which led to the
expansion of the Universe is the event known as the Big Bang.
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Box 2:
The Age of the Universe
Three different techniques are currently used to calculate the age of the Universe.
[See also http://www.astro.ucla.edu/~wright/age.html.]
1.
Velocity-distance relationships measured from galaxies
The Hubble constant, the value for the rate of expansion of the Universe
can be used to estimate the time that has elapsed since the formation of
the Universe at the Big Bang. This is estimated from the relationship 1/H 0.
Recent improved measurement of the parameters Hubble's constant, the
mass density of the universe, and the cosmological constant, yield an age
for the universe of 13.4 ± 1.6 billion years, which is a billion years younger
than other recent age estimates.See: Lineweaver, CH, 1999. A Younger
Age for the Universe, Science, 284, Number 5419, pp.1503-1507.
2.
Radioactive dating methods and models of galactic evolution
The ratio of the isotopes of the radioactive element Uranium when this
heavy element was synthesised can be used to estimate the age of the
Universe. Combining measurements of uranium isotope ratios with
models of galactic evolution allows us to infer an age for the Universe of
between 12 and 15 billion years.
Measurement of stellar age from uranium decay
Nature, 409, pp.691-692 (2001) © Macmillan Publishers Ltd.
R. CAYREL, V. HILL, T.C. BEERS, B. BARBUY, M. SPITE, F. SPITE, B. PLEZ, J.
ANDERSEN, P. BONIFACIO, P. FRANÇOIS, P. MOLARO, B. NORDSTRÖM & F.
PRIMAS
The ages of the oldest stars in the Galaxy indicate when star formation began, and
provide a minimum age for the Universe. Radioactive dating of meteoritic material and
stars relies on comparing the present abundance ratios of radioactive and stable nuclear
species to the theoretically predicted ratios of their production. The radioisotope 232Th
(half-life 14 Gyr) has been used to date Galactic stars, but it decays by only a factor of
two over the lifetime of the Universe. 238U (half-life 4.5 Gyr) is in principle a more precise
age indicator, but even its strongest spectral line, from singly ionized uranium at a
wavelength of 385.957 nm, has previously not been detected in stars. Here we report a
measurement of this line in the very metal-poor star CS31082-0018, a star which is
strongly overabundant in its heavy elements. The derived uranium abundance, log(U/H)
= -13.7 ± 0.14 ± 0.12 yields an age of 12.5 ± 3 Gyr, though this is still model dependent.
The observation of this cosmochronometer gives the most direct age determination of the
Galaxy. Also, with improved theoretical and laboratory data, it will provide a highly
precise lower limit to the age of the Universe.
3.
Models of stellar evolution
The luminosity of a star of a specified chemical composition is determined
by the supply of hydrogen to its core. We can therefore infer the age of a
star from its mass. The oldest stars calculated in this way are between 12
and 15 billion years old.
The three independent methods of dating all broadly agree, although the precision
claimed for the first model is currently thought to be the best estimate for the age of
the Universe.
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1.2
The Cosmic Microwave Background
One of the predictions of the Big Bang model for the origin of the Universe is that the
initial explosion was extremely hot and that the remnants of the initial fireball might
still be detected at the edges of the Universe. Support for this hypothesis came from
the discovery in the 1960s by Arno Penzias and Robert Wilson of the Bell telephone
Laboratories, of what came to be known as the Cosmic Microwave Background
(http://cfpa.berkeley.edu/darkmat/cmb.html). The discovery of the Cosmic Microwave
Background coincided with the work of some theoretical physicists who showed that
if the Universe began with a hot Big Bang, then the Universe should be filled with
electromagnetic radiation cooled from the early fireball to a temperature of around 10
degrees above absolute zero (~10°K). In subsequent years a large number of
measurements of the Cosmic Microwave Background at different wavelengths
yielded an intensity-wavelength plot which had the characteristics of black body
radiation at 2.73°K (2.73 degrees above absolute zero). This is the remnant of the
initial fireball of the Big Bang.
A difficulty with our measurements of the Cosmic Microwave Background is that they
are all derived from our local region of the Universe. Further confirmation of the Big
Bang Hypothesis has to come from the measurement of the Cosmic Microwave
Background from a more distant part of the Universe. This has now been achieved
by a very clever experiment reported in the journal Nature in December 2000.
Srianand and colleagues analysed light from a distant quasar, one of the most
luminous objects in the Universe. [Quasars are very bright objects which only exist
at immense distances. They are thought to be the cores of extremely distant, young
galaxies, that pump out huge amounts of energy. There are no nearby quasars.
Objects seen at immense distances are seen as they were billions of years ago,
because of the time it takes for their light to reach us. The presence of quasars
implies that the Universe was different in the past.] Their observations showed that
atoms of carbon, in the quasar were in an 'excited' fine-structure state, that is they
showed more energy than might be normally expected. The extra energy imparted to
these carbon atoms is thought to be due to the presence of the Cosmic Microwave
Background in the vicinity of the quasar. Calculations suggest that at the distance of
the measured quasar the Cosmic Microwave Background should be about 9°K.
Measurements reported in the study of Srianand and colleagues show that the
Cosmic Microwave Background is between 6°K and 14°K and in accordance with the
predictions of Big Bang theory. Reporting in the news and views section of Nature
John Bahall writes of this experiment 'the Big Bang theory has survived a crucial test,
… for … the theory would have been abandoned if astronomers had found that
clouds at earlier times had lower temperatures than predicted'.
More recently the quest has been for ever-precise measurements of the Cosmic
Microwave Background for these hold vital clues to the very early history of the
Universe. These measurements, however, have turned out to extremely difficult,
principally because of the very large amount of microwave radiation from other
sources, particularly from human activity, which has to be filtered out. Thus, in order
to avoid Earth-generated microwave radiation, a satellite was launched in 1989
carrying data to detect the cosmic microwave background. This experiment, known
as the COBE experiment, the COsmic Background Explorer satellite
(http://space.gsfc.nasa.gov/astro/cobe/ and
http://space.gsfc.nasa.gov/astro/cobe/ed_resources.html), produced an extremely
important result. High resolution temperature measurements with a sensitivity of a
few millionths of a degree were mapped over the entire sky and showed that the
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Cosmic Microwave Background is variable on the scale of ~30 millionths of a degree
K (see Figure 1). This observation is very important for it is this small variation to
which we owe our origins. A perfectly uniform Big Bang would have been
unacceptable because only heterogeneities, of the type now discovered by the
COBE experiment, are capable of permitting the formation of regions of matter, as
we now have in galaxies.
The most recent studies of the Cosmic Microwave Background have been based
upon balloon-borne microwave telescopes. The results from two such studies were
reported in 2000. In August 1998 the Maxima telescope
(http://cfpa.berkeley.edu/group/cmb/index.html) spent one night at 40 km above
Texas and later that year the Boomerang experiment was launched from Antarctica
and spent 10 days circumnavigating Antarctica. The results of these recent studies
have allowed an in-depth study of competing Big Bang hypotheses (see Box 3), and
provide confirmation that the Universe is 'flat’, i.e. finely balanced between expanding
for ever or collapsing back into a 'big crunch'.
Figure 1:
Box 3
Competing Big Bang Hypotheses
Alternative Big Bang models are concerned with the curvature of space. Over the
past 20 years there have been three competing models arguing that space may be
flat, positively curved, like a ball or negatively curved, like a saddle. The details of
these competing hypotheses are explained in the book by Silk, J. (1989) The Big
Bang, Freeman, p.90-93.
Increasingly, our understanding of the inflation of the Universe suggests that any
initial curvature in the Early Universe would be stretched to absolute flatness during
the initial massive expansion of the Universe. Before the idea of inflation came
along, cosmologists had the problem that if space is slightly curved at the start, it will
become massively curved during expansion. Inflation, however, stretches out any
wrinkles, just as blowing up a balloon flattens its surface.
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Now, new data from the study of the Cosmic Microwave Background support this
view. The results from the balloon-borne microwave telescope experiments Maxima
and Boomerang, launched in 1998, provide new observational evidence of variations
on the Cosmic Microwave Background.
The crucial cosmological parameter that determines the large-scale curvature of the
Universe is the density of the Universe — technically measured as Ω (omega), the
total energy density of the Universe, normalised to the critical 'closure' value.
Different values of Ω separates the positive curvature of an over-dense Universe
from the negative curvature of an under-dense one. In a flat universe the parameter
Ω = 1.0. New measurements from the Boomerang experiment reported in April 2000
showed that the value of Ω = 1.06 ± 0.06 and two weeks later the Maxima results
showed that Ω = 0.90 ± 0.07. These results strongly support a flat Universe and
support the inflationary scenario for the very early evolution of the Universe.
Details of the Maxima and Boomerang experiments are given in Peterson, J. (2000)
Universe in the balance, New Scientist Magazine, vol 168 issue 2269, (16 Dec) page 26.
1.3
The Early History of the Universe
The history of the Universe may be described in the following stages:

At the beginning of the Universe (ca. 13 billion years ago) all matter was in
one place at a single instant. This event, in cosmological parlance, is known
as a 'singularity', a term which describes the inference that an infinitely large
amount of matter is gathered at a single point in space-time.

At the Big Bang there was a huge expansion of matter, an expansion which
has continued ever since.

However, between 10-50 and 10-30 sec after the big bang there was a
particularly rapid expansion of the Universe. This process is known as
inflation. Later expansion slowed to a more normal rate.
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
During inflation the part of the Universe that we see today expanded by a
factor of 1060. At this point the universe was permeated with radiation and
subatomic particles (plasma/dark matter).

After 100 000 yrs conditions in the Universe were similar to those today inside
the sun. An almost uniform plasma of electrons and hydrogen and helium
ions filled the Universe. At this time the free electrons acted as a block to
photons — generated from the light energy generated in the Big Bang, and
prevented them escaping and rendering the early Universe opaque.

After 300 000 yrs temperature dropped to 4500°K gave rise to atomic matter
(hydrogen helium and deuterium) and because the electrons were now
removed (in the atoms), radiation streamed out and the Universe became
transparent.

As the universe continues to expand the radiation will appear to be derived
from a much cooler body, so that today the Cosmic Background radiation is
3.73 deg above absolute zero.
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2
Star Formation
In this section we consider the processes which lead from the Big Bang to the
formation of stars and galaxies. A principle which will become clear is that more
complex objects are progressively built from simple starting materials. This is also
true in the next section of this module on Planetary Formation. Thus during the Big
Bang the 'seeds' of matter were created. These eventually combined to form the
most primitive of all the chemical elements — hydrogen (chemical symbol H). In this
section we shall consider, amongst other issues, how the heavier elements were
constructed from the hydrogen atoms which formed close to the beginning of time.
2.1
'Lumpiness' of the Universe
The COBE experiment, designed to measure the remnants of the initial big bang
fireball at the edge of the Universe, also made another very important discovery.
Using a device known as a 'differential microwave radiometer'
(http://space.gsfc.nasa.gov/astro/cobe/dmr_synopsis.html) it became apparent that
matter and energy were not distributed evenly in the very early Universe.
Researchers found that the Cosmic Microwave Background was very slightly
variable on the scale of one part in 100,000. That is, there are very tiny
temperature fluctuations which can be measured by the differential microwave
radiometer. This unevenness in the distribution of matter and energy has been
described as the 'lumpiness' of the early Universe. The more scientific terminology
is that the Universe shows ‘anisotropy’. It is worth noting that elsewhere you will
read that the Universe is homogeneous or isotropic. The difference in viewpoint is
all a matter of scale. On a large scale the Universe is homogeneous and
everywhere has a temperature of 2.73°K at its edge. On a fine scale, however,
variations in temperature have been measured. It is this fine detail which is also
now seen to be important.
The significance of the variation in the intensity of the Cosmic Microwave
Background is that it shows how matter and energy were distributed when the
Universe was still very young. It is thought that these early inhomogeneities
subsequently developed into the regions in the present Universe where there is
matter (galaxies and galaxy clusters) and the regions from which matter is absent
(space). The early inhomogeneous distribution of matter also reflects an
inhomogeneous distribution of density. These initial density differences gave rise to
small differences in gravitational forces which began to draw matter together.
[Follow the link http://space.gsfc.nasa.gov/astro/cobe/dmr_image.html to view some
of the images of the early Universe measured by the COBE satellite. Some of the
images are contoured in microkelvins (µK), that is millions of a degree Kelvin, such is
the level of sensitivity of the measurements.]
2.2
Gravitational Collapse and the Formation of Stars
As the Universe began to evolve regions began to emerge in which concentrations of
the gases hydrogen and helium were higher than elsewhere. These formed the
basis for the development of what we now recognise as galaxies. Today galaxies
are huge concentrations of stars, and display a number of types based upon their
shape (see Box 4). They might also be thought of as the birth place of stars.
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Galaxies range in size from 80,000 to 150,000 light years in diameter. [A light year is
the distance travelled, at the speed of light in a year.] Within this huge volume,
galaxies contain very large numbers of individual stars — ca. 100 billion. There are
at least 10 billion galaxies in the Universe.
Box 4
Types of Galaxy
Galaxies are classified on the basis of their shape. There are two main types of
normal galaxy. These are the spiral and elliptical galaxies and are distinguished
according to their shape. Seyfert Galaxies are also a type of spiral galaxy which is
much more luminous than normal galaxies. Other galaxies belong to the irregular
type. Images of all these galaxy types can be viewed at
http://rst.gsfc.nasa.gov/Sect20/A2.html and at
http://www.aao.gov.au/images.html/general/galaxy_frames.html.
Radiogalaxies form a separate group of galaxies identified by their emission of
thermal and radio radiation.
Quasars (quasi-stellar objects) are very bright objects which only exist at immense
distances. They are thought to be the cores of extremely distant, young galaxies,
that emit huge amounts of energy, but which now are extinct. They therefore
represent a process which took place early in the history of the Universe but, which
has now ceased. Further details of quasars and their images may be found at
http://apod.gsfc.nasa.gov/apod/ap971206.html.
There is thought to be an evolutionary sequence to galaxies which has the pattern:
Quasars (oldest) → Radiogalaxies → Normal galaxies (youngest)
(i.e. decreasing redshift from oldest to youngest)
Galaxies formed, perhaps within the first million years of the history of the Universe, by
the inward gravitation collapse of the early matter of the Universe, thus enhancing the
difference between regions where there was matter and regions where there was
none. Inhomogeneities within galaxies, coupled with further gravitational collapse, are
the basis for star formation. A galaxy starts to form by the accumulation of hydrogen
gas in a very large cloud, called a nebula (see
http://www.aao.gov.au/images.html/general/emission_frames.html). At this stage the
gas cloud is huge compared with the later size of a galaxy. As matter aggregates in a
growing nebula the internal gravity draws in more gases. Eventually the nebula
develops localised 'clumps' of gas which continue to grow into even denser gaseous
bodies — stars. View the first three images at http://rst.gsfc.nasa.gov/Sect20/A2.html
to see the early stages in the development of a galaxy.
2.3
The Evolutionary Cycle of Stars
The key features of stars which are of interest to astronomers are their mass, their
luminosity, their surface temperature and the distance they are away from us. These
parameters are used to classify stars into types. More importantly they are also used
to place stars into an evolutionary sequence which describes their formation, life and
death. A widely used classification diagram for stars is the Hertzsprung-Russell
Diagram (the H-R diagram — see below) which is a plot of luminosity vs. effective
surface temperature. The luminosity of a star is a function of its radius and effective
temperature. (For more details see
http://www.worc.ac.uk/LTMain/Rowland/EIS2001/Lectures/Lecture03/Notes.html.)
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The Hertzsprung-Russell Diagram shows four main groupings of stars. These are
the:

Main sequence stars; these define a curved trend across the centre of the
diagram which displays a relationship between mass and luminosity, such that
stars with a high luminosity have a high effective temperature.

Red giants are stars which are cooler, but more luminous than stars on the
main trend — these are thought to be either dead stars or protostars.

Super giants are similarly stars which are more luminous, but much cooler
than main trend stars.

White Dwarfs are stars which are hotter and less luminous than main trend
stars and are thought to be dead stars radiating away their energy.
The cycle of birth-life-death for a star may be charted on a Hertzsprung-Russell
diagram.
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Most stars evolve onto the Main Trend and then into the Dwarf stage as follows:

Development of a large cloud of increasingly dense hydrogen (and helium)
gas and dust. This giant molecular cloud contains molecular hydrogen at low
temperatures (hundreds of °K). This is a nebula and within the nebula
aggregates of gas and dust form the beginnings of a star.

Temperatures rise, driven by gravitational collapse and a Protostar forms at
about 2000-3000°K.

The star subsequently evolves to be part of the main sequence of stars. The
precise position on the main sequence depends upon the mass of hydrogen
present in the star. At this point in its life temperatures in the core of the star
are as high as 107°K. Nuclear fusion takes place within the centre of the star
and helium is formed. At temperatures above 108°K He reacts through
nuclear fusion to form heavier elements. Whilst stars are part of the Main
Sequence developmental stage planetary formation may take place. (See
http://www.aao.gov.au/images.html/general/planetary_frames.html.)

The star eventually converts into a Red Giant and expands to up to 100 times
the diameter of the original star. Red Giants develop as the hydrogen in the
core is depleted and the helium derived from it tries to burn to carbon. A Red
Giant is sometimes described as a ‘bloated’ star.

The final stage of stellar evolution depends upon star size. Smaller stars end
up as the surviving cores of Red Giants which become greatly reduced in size
to the White Dwarf stage. Larger stars shed nearly all their remaining
gaseous matter and synthesize elements in a supernova (see
http://www.aao.gov.au/images.html/general/supernova_frames.html) — a
huge explosion marking the end of the life of the star. Within the dense
supernovae, neutron stars may form as the extremely dense endpoint in the
life of a star.
Further details of the process of star formation may be found on the following web
sites:

http://rst.gsfc.nasa.gov/Sect20/A5.html

http://www.astronomynotes.com/evolutn/evolutna.htm

http://map.gsfc.nasa.gov/m_uni/uni_101stars.html

http://www.worc.ac.uk/LTMain/Rowland/EIS2001/Lectures/Lecture04/Notes.html

http://www.worc.ac.uk/LTMain/Rowland/EIS2001/Lectures/Lecture05/Notes.html
2.4
Origin of the Chemical Elements
During the process of stellar evolution nuclear fusion reactions take place within a
star. These give rise to the formation of the chemical elements. Clues to the
processes involved may be deduced from the graph below, which shows the cosmic
abundances of elements (y-axis) plotted against atomic number (x-axis). The
abundances are measured relative to 1 million silicon atoms and the y-axis is a
logarithmic scale. In this case the data are for the solar-system and abundances
have been measured from the optical analysis of the solar spectrum.
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There are three important observations to be made from this graph:
1. The graph has (overall) a smooth trend from left to right, that is from light to
heavy elements. This indicates that abundances of elements in the solar
system are the greatest for the light elements and the least for the heavy
elements.
2. Superimposed upon the smooth trend described above is a smaller scale
irregularity such that elements with even atomic numbers have higher
abundances than those with odd atomic numbers.
3. Some elements have anomalous abundances. Hydrogen (H) and Helium (He)
and Iron (Fe) have anomalously high concentrations and the elements Lithium
(Li), Boron (B) and Beryllium (Be) have anomalously low concentrations.
It is thought that the early Universe consisted almost entirely of the element hydrogen,
with a small amount of helium present too. Hydrogen, therefore, is thought to be the
starting material from which all other elements have been built. This is consistent with
the very high abundance of hydrogen in the solar abundance profile. The process may
be thought of as a series of fusion reactions which weld together simple atomic nuclei
to build increasing complex atomic nuclei. The manner in which this is done depends
upon the internal temperature of the star and on its mass.
Early in star development hydrogen is utilised to manufacture the element helium.
As the hydrogen in the star is used up, the star contracts and its temperature rises so
that nuclear reactions can take place which permit the synthesis of the elements
carbon, nitrogen and oxygen, from helium (see
http://chemistry.ewu.edu/breneman/origin.htm). When the helium is almost
completely consumed the carbon and oxygen can be transformed into elements with
masses up to that of silicon. Increasing nuclear reactions, at higher temperatures
lead to the formation of elements with masses up to that of iron (Fe). Beyond this
point heavier elements cannot be formed by the process of nuclear fusion because
the temperatures required are higher than those found in stars.
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Elements heavier than iron are formed by the addition of neutrons which are absorbed
by the atomic nucleus. The very high abundance at the iron peak suggests that nuclei
of the element iron are formed more readily than they are used in the process of
neutron capture (see http://www.phy.anl.gov/ria/scicase/astro/astro_text.html).
The anomalously low concentrations of the elements Li, Be and B indicate that they
are by-passed in nuclear fusion reactions and their genesis seems to be explained by
the partial decay of heavier nuclei of the elements carbon and oxygen.
Differences in concentration between elements with even and odd atomic numbers
can be explained by the greater stability of atomic nuclei with paired neutrons. Thus
elements with even atomic numbers have the greater nuclear stability. This allows
their abundances to grow.
You may wish to follow up this section with visits to the following sites:

http://www.sprl.umich.edu/GCL/paper_to_html/
universe.html#ORIGIN_OF_ELEMENTS

http://map.gsfc.nasa.gov/m_uni/uni_101bbtest2.html

http://photon.phys.clemson.edu/wwwpages/StarLife.html and try the quiz at
the bottom of page

http://mineral.gly.bris.ac.uk/Mineralogy/elements/elements.html
If you are really keen you may wish to try

http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/
text_stel_6.html
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3
Planetary Formation and the Origin of the Earth
In the previous section it was shown that matter in the form of the chemical elements
formed during the process of star formation. In this section we discuss the processes
whereby this early-formed matter is assembled into planetary bodies. A key concept
here is the process of accretion — the progressive growth of larger bodies from
smaller fragments. Thus the 'story' of planetary formation is one in which a very large
number of small particles becoming progressively transformed into a small number of
larger bodies, which eventually become planets.
3.1
The Planets of the Solar System
Most of our knowledge of planetary evolution is drawn from the study of the solar
system. A wealth of helpful descriptive data about our solar system can be found at
http://www.hawastsoc.org/solar/eng/solarsys.htm. A similar site with a huge amount
of detail is http://www.ex.ac.uk/Mirrors/nineplanets/.
Some of the more important points are summarised in the table below:
The Terrestrial Planets
The Jovian planets
Body
Sun
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Distance1
0
0.39
0.72
1
1.15
5.2
9.2
19.2
30.1
39.5
Density2
1.41
5.43
5.25
5.52
3.95
1.33
0.69
1.29
1.64
2.03
Radius3
109
0.38
0.95
1
0.53
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9
4
4
0.18
1
Increasing Mean distance from Sun (Earth to Sun = 1.0)
2
Mean density of terrestrial planets greater than that of Jovian planets
3
Radius of terrestrial planets less than that of Jovian planets (Earth = 1.0)
3.2
The Stages of Planetary Formation
Our knowledge of the process of planetary formation comes from a number of
diverse sources which include:

The physical observation of stars

The physical properties of the planets of the solar system

The study of meteorites

Theoretical calculations
Broadly, four stages can be identified in the process of planetary formation.
1. The gravitational collapse of a star leads to the formation of a core to the gas
cloud and the formation of a huge rotating disc of gas and dust, which
develops around the gas core. A star such as Beta Pictoris shows a central
core of this type, with a disc of matter rotating around the core. Beta Pictoris
is thought to be a young star showing the early stages of planetary formation.
(See http://www.ifa.hawaii.edu/faculty/jewitt/Origins-bpic.html.)
2. The condensation of the gas cloud and the formation of chondrules.
Chondrules are small rounded objects found in some meteorites. For some
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general background on meteorites see
http://www.ex.ac.uk/Mirrors/nineplanets/meteorites.html. The presence of
chondrules gives rise to a special class of meteorites known as chondrites.
(For images of chondrites see one or more of the following sites:

http://pds.jpl.nasa.gov/planets/captions/smallbod/chndrite.htm

http://www.labenne-meteorites.com/98044.htm

http://www.labenne-meteorites.com/murchison.htm.)
For example, the Allende meteorite is chondrule-rich and contains minerals
rich in the elements Ca and Al, and Ti and Al, minerals which are unlike
terrestrial minerals. It also include metallic blobs of Os, Re, Zr. The chemistry
of these unusual minerals suggest that they are early solar system
condensates.
3. The accretion of gas and dust to form small bodies between 1-10 km in
diameter. These bodies are known as planetesimals. They form initially from
small fragments of solar dust and chondrules by the processes of cohesion
(sticking together by weak electrostatic forces) and by gravitational instability.
Cohesion forms fragments up to about 1 cm in diameter. Larger bodies form
by collisions at low speed which cause the material to stick together by
gravitational attraction.
Support for this view of the process of accretion comes from a region on the
edge of the solar system known as the Kuiper Belt, where it is thought that the
accretionary 'mopping up' has failed to take place. For more detail about the
Kuiper Belt, see the web sites:
 http://www.ifa.hawaii.edu/faculty/jewitt/kb.html
 http://www.hawastsoc.org/solar/eng/kuiper.htm
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4. More violent and rapid impact accretion. The final stage of accretion has been
described as 'runaway accretion'. Planetesimals are swept up into well
defined zones around the sun which approximate to the present orbits of the
terrestrial planets. The process leads eventually to a small number of large
planetary bodies. Evidence for this impacting process can be seen in the early
impact craters found on planetary surfaces
(http://www.hawastsoc.org/solar/eng/mercury.htm#views,
http://www.seds.org/billa/pics/Luna2.jpg).
An explanation of the type given above for the origin of the planets in the solar
system is supported by mathematical simulations which show how accretion works
by the progressive gathering together of smaller particles into large. It also provides
an explanation of the differences between planetary bodies in the solar system and
explains the differences between the heavier terrestrial planets close to the sun, and
the lighter, more gaseous planets situated at a greater distance.
3.3
The Search for Planets in other Solar Systems
The discovery of planets from outside the solar system is a new and fast-growing
field of research. As of July 2001 there are 67 planets recorded from outside our
own solar system. The first such planet was reported in the scientific journal Nature
as recently as 1995 by Michel Mayer and Didier Queloz from the University of
Geneva. This first discovery was around star 51 Pegasi. Shortly afterwards, in 1996,
Geoffrey Marcy and Paul Butler from the USA reported a planet around the star
Upsilon Andromedae — a discovery which subsequently revealed an entire planetary
system associated with this star.
3.3.1 Finding Extrasolar Planets
Compared to the intense brightness of a star, planets are invisible and cannot
therefore be directly observed. It has long been known, however, that the presence
of planets can be inferred by the gravitational pull they exert on their parent star.
This should produce a tell-tale wobble in the motion of the star, which can be used to
reveal the presence of an unseen planet.
Success in detecting wobble in the motion of a start has come from the analysis of
the light emitted by the star. This allows us to measure the speed of the star.
Spectroscopy — the analysis of starlight — splits up the light from a star into a
spectrum and a measure of the shift of the absorption lines (the Doppler Shift) is
used to calculate how fast the start is moving. [Visit http://exoplanets.org — go to
'Public Information', then 'The Doppler Detection Method'.] Stars which show a
changing velocity are candidates for stars with planetary systems. If the change in
velocity has a regular periodicity which is shown to be in accord with Kepler's Laws of
planetary motion, then a planet may be inferred. By 1995 astronomers had improved
the sensitivity of their spectroscopic methods such that the precision of velocity
measurements had been increased from about 1 km/sec down to 20 m/sec. This
paved the way for the first observations of a planet around star 51 Pegasi.
A second technique for detecting extrasolar planets is the observation of changes in
star brightness. If a planet passes over the surface of the star, in certain conditions,
the brightness of the star will be reduced. In the autumn of 1999 a 2% dimming of
the star HD 209458 was observed at exactly the time predicted from calculations
made of the orbit of an inferred planet. These observations provided important
confirmatory evidence of results obtained using the Doppler method. [Visit
http://exoplanets.org — go to 'Public Information', then 'Various Planetary Detection
Methods'.]
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3.3.2 Discoveries to date
Most of the recorded extra-solar planets to date have a mass close to that of Jupiter.
Such planets are large gaseous planets, with a mass of about 1% of the sun. In
March 2000 planets with a smaller mass were first detected and two planets with the
mass of Saturn were found orbiting the stars HD 46375 and HD 16141.
Close analysis of the data from the star Upsilon Andromedae, 44 AU (astronomical
units, i.e. earth-sun distances) away showed that there were three planets present.
The innermost is ¾ the mass of Jupiter, 0.06 AU from the star and orbits the star in
just 4.6 days; the middle planet is twice the mass of Jupiter, orbits at 0.3 AU from the
star and takes 242 days to orbit. The outer planet has a mass four times that of
Jupiter, is a distance 2.5 AU from the star and takes 3.5-4 years to orbit the star.
These observations are of particular interest to astronomers and initially were a great
surprise for these observations are very different from the pattern of planets in our
own solar system. It was not expected that planets orbiting other stars would be
large and very close to their parent star.
Details of planets associated with the star Upsilon Andromeda are at
http://www.jtwinc.com/extrasolar/mainframes.html, select 'Planets of normal stars',
select 'Upsilon Andromedae'.
3.3.3 For further information:

http://www.astronomy.com/home.asp — look at Feature Stories

http://www.ex.ac.uk/Mirrors/nineplanets/other.html
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4
And Finally...
To put all the topics of this module — The Origin of the Universe, Star Formation and
Planetary Formation — into some perspective, and to gain an insight into the
enormity of the scales involved in the formation of the solar system view the web site:
http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/index.html and
explore the 'powers of ten' animation.
4.0.1 Reading and Further information
Boss, A. (1996) Extrasolar Planets. Physics Today, September 1996, 32-38.
Clarke, S. (2001) Out of this world. New Scientist, 169, issue 2273 (13 Jan 2001),
page 44.
Marcy, G.W. & Butler, R.P. (1998) Giant Planets Orbiting Faraway Stars, Scientific
American Presents, 9(1) Magnificent Cosmos.
4.0.2 Other useful websites
Astroweb: http://webhead.com/wwwvl/astronomy/
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